This invention relates to the field of integrated circuit fabrication and particularly to the doping of compound semiconductors.
Conventional integrated circuit technologies have been successful in producing a wide variety of circuits and devices. Recently, integrated circuits and devices also have been developed using GaAs and other III-V compound semiconductors because of their speed advantage over comparable Si devices, because of the wide range of bandgaps available for optical signal processing, and because of the possibilities for high temperature operation. In the compound semiconductors, however, the limits of existing fabrication technologies are being reached, and new techniques are needed to fully exploit their potential. In particular, exploitation of techniques to fabricate truly three-dimensional device and circuit structures has the potential to dramatically lessen the constraints of conventional processing and device design. Further advantages in device yield and process control can be expected by integrating device processing with epilayer growth, using the controlled environment of the growth apparatus. This would lead to an in situ 3-D device capability with enhanced design freedom and device yield.
Presentlly, compound seimconductors are selectively doped using ion implantation. The surface to be doped is either a polished GaAs wafer cut from GaAs boule, or is first formed epitaxially by a process such as molecular beam epitaxy (MBE). In the latter case the semiconductor material is then removed from the high vacuum MBE-chamber and a mask of photoresist or other ion-blocking materials such as Si.sub.3 N.sub.4 formed on its surface. The masked semiconductor is placed in an ion implantation chamber and bombarded with dopant ions. Those areas of the semiconductor's surface which are not covered by the mask receive the dopant.
If uniform (non-selective) doping of the entire surface is desired, the surface can be doped in situ in the same chamber used to grow the surface. If the dopant impinges on the growing layer, the doping density will be proportional to the dopant flux divided by the growth rate of the epitaxial material. Dopant variations in the direction of growth (perpendicular to the epitaxial surface) may be caused by varying the dopant flux with time. In extreme cases, the dopant atoms may be deposited in a single plane. For example MBE growth of GaAs can be suspended, and dopants such as Si, Ge, Be, and Sn introduced into the MBE chamber. A submonolayer of the dopant is deposited in situ, and when MBE growth of the GaAs is resumed the dopant is incorporated into the growing layer. The doping is accomplished without removing the semiconuctor from the high vacuum chamber. However, the entire surface is doped in a non-selective manner.